the beam strikes the ST at angles less than 30° from the axis. However, the ST is moving at 7.5 km/s and the beam is moving at a high velocity. Thus, the time that the ST spends in the beam is < 44 seconds (the value for a stationary beam) and depends on the details of the geometry. For long exposures, the effect of the specular reflection beam will be reduced. The effect of the SPS on the ST will be to require either 1) a zone of avoidance 60° x 120°, or 2) detailed modeling of the effects for each target. Either option will require detailed simulations to provide a realistic assessment of the impacts. However, it appears likely that contamination by the specular reflection beam may exclude observations of certain regions of the sky during portions of the ST orbit. Ordinarily, this will not be a major problem since the affected regions can be scheduled for observations when the SPS is favorably located (until Europeans, the Soviet Union, the Chinese, etc. put up their own SPS, which will fill the entire equator with satellites). However, very long exposures (with an elapsed time of about 30 hours) or observations which must occur at specific times may still be adversely affected. A typical path of an SPS satellite as seen from the ST is shown in Figure 2. This complicated path is important for bright object avoidance. The specular and diffuse reflections from the SPS satellites are so bright that they cannot be imaged on an operating sensor without risk of damage to the sensor. To ensure that this does not happen, the ground systems will have to calculate the positions of all the satellites as seen from the ST whenever the ST is observing a target close to the equator. This will greatly complicate the ground system activity associated with scheduling, constraint checking, and command generation. It may also force the ST into a zone of avoidance mode to keep the ground systems costs under control. This would exclude the 12° band around the equator and would also complicate star tracker usage. There will undoubtedly be a cost impact although further study is required before a realistic assessment of the cost and complexity of the impacts can be given. Ground communications for the ST are via the Tracking and Data Relay Satellite System at the following carrier frequencies: 2.106 GHz, 2.255 GHz, and 2.287 GHz. These frequencies are uncomfortably close (within 200 to
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